In a subsonic aircraft having an externally mounted engine, for example, a gas turbine engine mounted below a wing by a pylon, aerodynamic drag due to freestream airflow over the nacelle of the engine can typically represent approximately 4% of the total engine thrust output. Any reduction in this aerodynamic drag can result in a significant saving in the amount of fuel consumed.
Accordingly, a desired function of an engine nacelle is to provide a lightweight housing for the aircraft engine which produces relatively low aerodynamic drag. An example of a prior art low drag nacelle is disclosed in U.S. Pat. No. 3,533,237, issued to G. R. Rabone et al, assigned to the present assignee, and incorporated herein by reference.
The aerodynamic drag due to a nacelle is determined by the pressure distribution and a dimensionless friction coefficient C.sub.f, known to those skilled in the art, over the outer surface of the nacelle over which the freestream air flows during aircraft flight. It is known to those skilled in the art that reduced aerodynamic drag exists where the surface pressure distribution promotes a laminar boundary layer over the nacelle outer surface without any boundary layer separation thereof. The friction coefficient C.sub.f, and thus aerodynamic drag, have reduced values when a laminar boundary layer exists.
Also known to those skilled in the art is that where the boundary layer along the nacelle outer surface transitions from laminar to turbulent, the friction coefficient C.sub.f, and thus aerodynamic drag, have increased values. Accordingly, it is desirable to provide a nacelle which promotes a surface pressure distribution effective for increasing the extent of laminar boundary layer flow, reducing the extent of turbulent flow and avoiding boundary layer separation.
A nacelle is typically an annular member which houses an aircraft engine, such as a gas turbine engine. Unlike a wing which extends longitudinally and has upper and lower surfaces designed for maximum lift and reduced drag, a nacelle extends circumferentially and has an outer surface designed primarily to house an engine and reduce drag.
However, in both a nacelle and a wing the pressure distribution over the surfaces thereof is a significant factor in determining the extent of laminar and turbulent airflow thereover. In a wing, for example, the pressure distribution is dependent on the contours of the leading and trailing edges and the upper and lower surfaces. A change in any contour affects the entire pressure distribution over the wing.
In a nacelle, in contrast, the pressure distribution is primarily affected by the contours of the leading and trailing edge regions and the outer surface. The inner surface of the nacelle has little interaction with the freestream airflow, and therefore has less affect on the pressure distribution.
Furthermore, inasmuch as a nacelle is typically mounted to an aircraft near a fuselage, pylon or wing, the pressure distribution over the nacelle can also be affected by the presence of these adjacent structures. A change in any contour of the elements of the nacelle and the presence of adjacent structures affects the entire pressure distribution over the outer surface of the nacelle.
Past attempts at maintaining and extending laminar flow on wings and nacelles have involved the use of active control devices. An active control device requires an auxiliary source of energy to cooperate with the surface for energizing or removing the boundary layer for maintaining laminar flow and preventing boundary layer separation.
For example, boundary layer suction or blowing slots or holes disposed in the surface to be controlled are known in the art. The slot is connected to a pump by internal ducting and is effective for reducing or preventing turbulent flow, and thereby maintaining laminar boundary layer flow. However, the additional weight and energy required to power the active control device typically offsets advantages due to reduced aerodynamic drag.
Accordingly, it is one object of this invention to provide an improved nacelle for housing an aircraft engine which is effective for reducing aerodynamic drag during aircraft operation.
Another object of this invention is to provide an improved nacelle which does not require an active device for reducing aerodynamic drag.
Another object of this invention is to provide an improved nacelle having increased areas of laminar flow and decreased areas of turbulent flow thereover.
Another object of this invention is to provide an improved nacelle having a profile effective for controlling surface pressure distribution thereover for reducing aerodynamic drag.